Musk tweeted Sunday that a “kid-size submarine” meant to help the boys navigate narrow, flooded passageways on their way to safety is on the 17-hour trip to Thailand from Los Angeles. The delivery announcement comes on the second day of an operation to remove the boys. Sunday, rescuers successfully navigated the first batch of four boys through the perilous escape route.

Responding to a question on Twitter, the entrepreneur and engineering mastermind said “divers who have made the passage,” believe the pod is small enough to fit through the most narrow portions of the cave. “We also made an exact replica that is inflatable, so that the entire path can be tested without risk of blockage,” he added.